diff --git a/var/spack/repos/builtin/packages/curl/package.py b/var/spack/repos/builtin/packages/curl/package.py
index 8923423efb1..26be294245f 100644
--- a/var/spack/repos/builtin/packages/curl/package.py
+++ b/var/spack/repos/builtin/packages/curl/package.py
@@ -129,7 +129,7 @@ def determine_variants(cls, exes, version):
         for exe in exes:
             variants = ""
             curl = Executable(exe)
-            output = curl("--version", output=str, error="str")
+            output = curl("--version", output=str, error=str)
             if "nghttp2" in output:
                 variants += "+nghttp2"
             protocols_match = re.search(r"Protocols: (.*)\n", output)